My suggestion is...

Not knowing the strength of bandit camp/ monster lairs unless you have a ranger and/or the spell of seeing.

A ranger only, would give you 30% accuracy

A spell of seeing only, would give you 70% accuracy

Having a ranger and casting the spell of seeing would give you 100% accuracy.

If you had no ranger and had no, or not used, the spell, then and found yourself up against a unbeatable foe, you could play British bulldog, which would involve getting your units to the enemy's back line of tiles to escape the battle.